1. Field of the Invention
The present invention relates to a transmitter and a transmission method thereof, more particularly to a transmitter for wireless communication and a transmission method thereof.
2. Description of the Related Art
A wireless communication system transmits data through a channel with a predetermined frequency band. In order to reduce interference between wireless communication systems using different frequency bands, the energy outside of the frequency band used by each communication system is limited.
Referring to FIG. 1, a conventional wireless communication system that operates in a predetermined frequency band includes a transmitter 1 and a receiver (not shown) The transmitter 1 converts an input signal into an output signal, and transmits the output signal to the receiver. The transmitter 1 includes a first filter 11, a digital-to-analog (D/A) converter 12, a second filter 13, a mixer 14, a power amplifier 15, and an antenna 16. The first and second filters 11, 13 are used for making the bandwidth and energy of the output signal correspond to bandwidth and energy limitations of the predetermined frequency band. The mixer 14 is used for making a center frequency of the output signal correspond to a center frequency of the predetermined frequency band.
When the power amplifier 15 has a nonlinear effect, intermodulation will cause out-of-band energy of the output signal to increase. This problem is exacerbated by increases in the bandwidth and energy of the output signal. Additional information on intermodulation may be found on pages 14-25 of “RF Microelectronics” by Behzad Razavi. When the bandwidth and energy of the output signal are relatively small, such a nonlinear effect of the power amplifier 15 will not cause problems. However, if the energy of the output signal is increased to enlarge the range of coverage, or if the bandwidth of the output signal is increased to increase transmission data capacity, the energy of the output signal outside the predetermined frequency band (i.e., out-of-band energy of the output signal) becomes excessive, such that interference occurs with other wireless communication systems using different frequency bands.
Since there is no filter positioned after the power amplifier 15 to attenuate the out-of-band energy of the output signal and reduce the out-of-band energy of the output signal by decreasing the gain of the power amplifier 15, the coverage range of the wireless communication system is undesirably diminished.